1. Field of the Invention
This invention relates to a shipping container for a nuclear fuel assembly, and in particular, to such a container for nuclear fuel assemblies which have a plurality of fuel rods held in a hexagonal array by a plurality of grids spaced longitudinally along the fuel rods. The invention also relates to a hold-down device for securing the bottom nozzle of the nuclear fuel assembly.
2. Background of Information
In the shipping and storage of nuclear reactor fuel elements and assemblies, which contain large quantities and/or enrichments of the fissile material, U.sup.235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments (i.e., weights and weight percent U.sup.235) for each fuel assembly design. In order for a new shipping container design to receive licensing, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC Rules and Regulations, including those defined in 10 CFR 71 which is incorporated herein by reference. These requirements define the maximum credible accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the subcriticality of the fuel assemblies therein.
U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention and which is incorporated herein by reference, discloses a shipping container for transporting two conventional nuclear fuel assemblies having a square top nozzle, a square array of fuel rods and a square bottom nozzle. The container includes a support frame having a vertically extending section between the two fuel assemblies which sit side by side. Each fuel assembly is clamped to the support frame by clamping frames which each have two pressure pads. This entire assembly is connected to the container by a shock mounting frame and plural shock mountings. Sealed within the vertical section are at least two neutron absorber elements. A layer of rubber-cork cushioning material separates the support frame and the vertical section from the fuel assemblies.
The top nozzle of each of the conventional fuel assemblies is held, along the longitudinal axis thereof, by four longitudinally attached bolts at the four corners of the square top nozzle. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held, along the longitudinal axis thereof, by a bottom nozzle spacer which holds the chamfered end of the bottom nozzle.
This and other shipping containers (e.g., RCC-4 for generally square cross-sectional geometry pressurized water reactor (PWR) fuel assemblies) used by the assignee of the present invention are described in certificate of compliance No. 5450, Docket 71-5450, U.S. Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of Nuclear Material Safety and Safeguards, Washington, D.C. 20555, which is incorporated herein by reference.
In nuclear reactors of the type originally designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies. Each of these fuel assemblies includes a plurality of fuel rods held in an organized hexagonal array by a plurality of hexagonal grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimble tubes.
Subsequently, the Soviet-style fuel assemblies were redesigned by the assignee of the present invention in order to provide, for example, more reliable operation. The guide thimble tubes of the redesigned fuel assemblies extend above and below the ends of the fuel rods and are attached to top and bottom nozzles, respectively. Such fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles. These fuel assemblies may contain U.sup.235 concentrations of up to about 4.80 to 5.00 weight percent U.sup.235. Under normal manufacturing conditions, the dimensions of the fuel assemblies may vary. For example, the dimensions of the six sides of the hexagonal array may differ by about .+-.2.0 mm between individual fuel assemblies.
The top nozzle of the fuel assembly includes a cylindrical outer barrel, a cylindrical inner barrel and a hub. The outer barrel forms a first end of the top nozzle at the top of the fuel assembly. The inner barrel, which has a diameter smaller than the outer barrel, is attached to the hub, which forms a second end of the top nozzle opposite from the first end. The outer barrel has a shoulder facing the second end. The inner barrel telescopes into the outer barrel. The hub interfaces the plurality of fuel rods at the second end.
The relatively heavy (e.g., 70 pounds) top nozzle is susceptible to transportation induced damage to the guide thimble tubes. For example, during normal transportation, vibration in the top nozzle inner barrel may be detrimental to the guide thimble tubes. Because of the unique design of the fuel assembly, which allows movement of the outer barrel along the longitudinal axis of the fuel assembly with respect to the relatively smaller inner barrel, it is not feasible to position adjustable hardware for securing the top nozzle in order to provide the necessary supporting restraint of the fuel assembly during shipment thereof.
The bottom nozzle includes a longitudinally extending recess formed by a hexagonal barrel, a spherical taper, and a cylindrical barrel which has a diameter smaller than the hexagonal barrel. The spherical taper forms a tapered bore within the longitudinally extending recess tapering toward the bottom end. The spherical taper, also, forms an internal shoulder between the hexagonal barrel and the bottom end.
There is a need, therefore, for an improved shipping container for a nuclear fuel assembly having a double-barrelled top nozzle. There is also a need for an improved shipping container for a nuclear fuel assembly having a double-barrelled bottom nozzle.
More particularly, there is a need for such a container for a nuclear fuel assembly having a hexagonal geometry.
There is an even more particular need for such a container which accommodates for manufacturing tolerances in the hexagonal geometry.
There is another more particular need for such a container for a nuclear fuel assembly including a top nozzle having an outer barrel and an inner barrel of smaller diameter which telescopes into the outer barrel.
There is yet another more particular need for such a container for a nuclear fuel assembly including a bottom nozzle having a longitudinally extending recess formed by a hexagonal barrel, a spherical taper, and a cylindrical barrel having a diameter smaller than the hexagonal barrel.
There is still another more particular need for such a shipping container for transporting high enrichment fuel assemblies.